THD: Yello Jackets

I just bought a pair of THD Yellow Jacket tube convertor sockets with EL84s
[too tall!]. I plan to upload the instructions. My ancient Kay amp with 2 6V6's
has been cutting up and down in volume; I think the tubes are totally
mismatched and are shot to hell from my amp-cranking experiments with a speaker
isolation cabinet.

These replace common tubes such as 6V6, 6L6, or 6550, by an EL84 pair in
class A operation. Can I only plug in a single EL84?

If you start with a Marshall with 4 power tubes, you can pull all 4, and put
in two Yellow Jackets with EL84s, resulting in about 1/4 power -- 1/5, the instructions
say (20%). A 100 watt Marshall becomes a 20 watt Marshall.

I've been doing so much online research. First I did a lot of hardware
research, in the days of the DigiTech 2101 and the Harry Kolbe Silent Speaker
R/L load. Then I got rid of practically all my gear, just for psychological
reasons -- to clear the drawing board and start with a completely fresh mental
slate, and to do pure literature research to figure out what pieces are
available and how to put them together in an elegant way. That was about 1996.

Now I have researched all the possibilities and I'm looking forward to
buying gear and experimenting. I figure I'm going to have to invest in a
variety of speakers, cabs, amps, power-tube products, and effects. Looks like
an oversized custom flight case could be $600, to form a giant isolation box
(doghouse), or mini isolation booth. I also live now in a house that has a
cement-block darkroom. And now, totally unlike 1996, there is a fair selection
of tube power amps in the 0-10 watt range. And now WeberVST has an established
line of great speakers, and MP3 and hard drive recording technology is mature,
and everyone's one the Web. And digital cameras are established.

And ToneFrenzy.com clearly has the start of a great thing going, though they
*suck* at the moment at miking amps. So I'm feeling eager to upload some
samples that will impress people. I might actually solicit .wav files from good
guitarists as input data for my power-tube processing transforms.

When I was cranking my Kay and melting down the old, mismatched power tubes,
I certainly proved one thing: the "re-amp" approach with a short
looping clip kicks *ass* for dialing-in a tone. I sure grok the studio logic of
always recording the straight guitar as well as the amp tone, during the
original take. It is incredibly powerful, infinitely powerful, being able to
run this source signal later, using a computer or whatever, through any
black-box processors you want. A miked cab driven by a power tube is merely
another abstract I/O transform... especially I'm reporting the *leisurely*
sense. No more nightmare of trying to strum and turn knobs at the same time!

ToneFrenzy.com has also discovered the immense power of this approach, for
mass-production comparison of effects. It *really helps* to have a consistent
self-playing guitar signal so you can use both hands to dial in a Tone, compare
mics, compare speakers, etc. You can record the result, say, of micing each
speaker model, save a library of those tones to hard drive in an organized database,
then A/B any Tone, all via WinAmp -- all driven by the same input signal.

I want to buy and try things like the Black Jack tube convertors which
enable plugging a certain spec'd preamp tube into a power-tube socket.

Yello Jackets convert your Class AB power section to
cathode-biased Class A using EL-84 tubes. It works exactly as advertised. I
installed a matched duet in a 1972 Fender Deluxe Reverb that had been
blueprinted to Blackface specs. I bought this amp used, with the original (RCA,
GE) tubes, so I had a good benchmark for evaluating the Yello Jackets. I really
love the sound of this amp with the Yello Jackets more than I did with the GE
6V6s. I like a detailed surgical tone. The 6V6s always seemed to splatter a
bit. Yello Jackets have greatly tightened up the bass and yet the high end is
not strident. I had feared that the brighter EL84s night sound harsh with the
Fender front end. The midrange signature is now much more forgiving of my pedals
(MESA V-Twin, TS-808 and DM-2 analog delays). I think the Yello Jackets are an
improvement over the somewhat trashy Fender output stage. I might buy these for
other amps. I am curious as to how one of these would sound with the THD
Uni-Tube amp. This could make a 10 watt class A amp. My Yello Jackets have good
reliability. I ordered these direct from THD, and they answered several of my
questions in great detail before I placed the order.

See also

thdunivalve.htm - THD UniValve
head with 1 power tube, integrated Hot Plate, and Line Outthdhotplate.htm - THD Hot Plate power attenuator
with volume fine-adjust, Line Out jack with Level control, and high-level Bass
and Treble boost switches

THD Official Email Information about Yellow Jacket Convertors

THD Electronics, Ltd. announces its newest line of products: the Yellow
Jacket Converters. Yellow Jackets are a type of specialized adapters which
permit the use of EL84/6BQ5 power tubes in place of 6V6, 6L6, EL34 and 7591
types.

Simple Class-A Operation from Any Amp - Yellow Jacket Converters not
only rearrange the pin locations of the tubes, but also provide the necessary
current limiting on the screens and cathode as well as blocking the amplifier's
grid bias voltage, while configuring the EL84 in a Class-A, self-adjusting
cathode-bias circuit. In other words, there are no adjustments to make and no
modifications necessary, you simply plug the Yellow Jackets into the amp's
output tube sockets, (in cathode-bias amplifiers) screw the ground wires under
one of the output tube socket mounting screws, plug the provided EL84s into the
Yellow Jackets, turn the amp on and play.

Like Getting A Whole New Amplifier - In Champ and Deluxe type amplifiers,
the YJS's output is smooth, strong and more even than with the stock 6V6s. When
the YJS Converters are installed in larger amplifiers intended for 6L6 or EL34
output tubes, the high plate and screen voltages are reduced as well as current
limited to protect the EL84 and to drop output power. When a pair of YJSs are
used in a Marshall 50 watt or similar amplifier, the overall power drops to
about 20 watts and takes on more of the character of an old Vox AC30. The
distortion is smoother and more even and the output is both substantially
reduced and more compressed due to the Class-A nature of the Yellow Jackets. In
a 100 watt amp like a Twin Reverb or Marshall 100, either two or four YJSs can
be used. If only two are used and the other two sockets are left empty, the
output is dropped to about 20 watts. With four in place, the amp puts out about
40 watts and takes on a whole new warmth and richness.

In the cases of the YJ7591 converters, the overall output power remains the
same, but the sound becomes fuller and the extinct 7591 are replaced with
common and inexpensive EL84 tubes. In old Ampeg Reverb Rockets and the like,
you experience a warmth and sweetness that you could never get with the now
unavailable 7591 tubes.

No Adjustments or Modifications, Change At Will - As the Yellow
Jackets ignore the amplifier's grid bias voltage, it is possible to take them
in and out at will to adjust you power level and sound for the individual gig
or session. For an interesting blend of sounds, you can even exchange two of
the output tubes in a four tube amp for Yellow Jacket Converters and leave the
other two as 6L6s or EL34s. This blends the sound of both and lets you run a
combination of Class-A and Class-AB.

Safe For All Amplifiers and Transformers - All Yellow Jacket
Converters internally limit voltage and current they put no extra strain on
amplifiers' power and output transformers or internal components. Using the
YJSs in higher powered amplifiers actually reduces the strain on the
transformers and internal components because it reduces the overall wattage of
the system. Furthermore, the heater (or filament) current of the EL84 is lower
than that of the 6V6, 6L6, EL34 or 7591, so the Yellow Jacket will reduce the
strain on the filament windings of the power transformer as well.

THD's Yellow Jackets in the Press

Guitar World "... the next best thing to getting yourself a new
amp." May, 1996

"What do you do if you're tired of the way your amp sounds, but don't
have the bucks to buy a new one or to have some high-priced amp surgeon modify
its innards?

"Fortunately, THD has come up with the Yellow Jacket, a cost-effective,
non-permanent amp modification that allows you to replace your existing power
tubes with EL84s without rebiasing the amp. The concept is similar to that of
12AX7 preamp modifications like the Soldano Hot Mod, where you just plug an
adapter and tube into the socket and you're ready to go. But unlike preamp
adapters, which simply boost your amp's gain, the Yellow Jackets significantly
alter the tonal and response characteristics of your amp.

"The Yellow Jackets are available in four different versions: a set of
four adapters for EL34- or 5881-based 100-watt amps; a pair for EL34- or
5881-based 50-watt amps; a pair for 6V6-based amps; and a pair for 7591-based
amps. We tried the Yellow Jackets in a 100-watt Marshall Plexi reissue. Because
four EL84s provide about 50 watts of output, the amp wasn't quite as loud. The
treble response was enhanced, the midrange somewhat thicker, and the amp
produced rich, glassy distortion and sparkling clean tones similar to those of
a Vox AC30. The Yellow Jackets transformed the Marshall into a totally
different amp. Best of all, it took less than five minutes to install the
adapters.

"THD's Yellow Jackets are the next best thing to getting yourself a new
amp. Costing about the same as a good preamp or distortion pedal, they're the
way to go if you want to explore new tones on a budget."

Vintage Guitar "... too hip!" May, 1996

"THD Electronics, Seattle, has a device called the Yellow Jacket, which
enables you to switch the output tube of your amplifier by using an external
socket adapter that plugs into an amp's existing tube socket. These adapters
come in several versions; they essentially allow you to use EL-84 output tubes
in place of 6V6s, 6L6s, EL-34s and the extinct 7591 vintage Ampeg tube. No
re-biasing is required because it automatically makes your amp go into simple
Class 'A' operation. It's especially nifty if you can't find good (or the
correct type) tubes for your current amp, or if you simply want to change its
sound. List price is $60 each, tube included - too hip! You can also use two of
these things and leave the stock tubes in two of the amp's own sockets (in a
100-watt Marshall, for example), giving you a combination of both Class 'A' and
Class 'AB.'"

Guitar Player "... increase amplifier flexibility" May, 1996

"THD's Yellow Jacket converters ($60 each) increase amplifier
flexibility by allowing you to run EL84/6BQ5 output tubes in any amp that
normally uses 6V6s, 6L6s, EL34s or 7591s. They simply pop into the tube
sockets, instantly reconfiguring the EL84s to Class A self-adjusting cathode
bias. An EL84 is included with each converter."

THD Electronics, Ltd.
1925 8th Ave.
Seattle, WA 98101
(206) 292-5500

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